The present invention relates to barcode reading techniques, and more particularly, to a barcode scanning system having a compensation circuit for preprocessing a derivative signal response so as to prevent a decrease in sensitivity that could otherwise result from the differences in signal amplitude and frequency.
As illustrated in FIGS. 1 and 2, while reading a barcode 20 by a scanner 10, a mirror 11 in the scanner projects a light beam 15 onto the barcode 20 so as to obtain reflected light from the barcode 20 for generating a signal response by a photodetector (e.g., a photodiode 12). When the mirror 11 oscillates to implement the scanning function, the light beam 15 moves across the barcode 20 to collect the information encoded in the bars and spaces of the barcode 20. The photodiode 12 generates a photocurrent as the signal response to the light reflected from the barcode 20. A low pass filter (LPF) 30 rejects high frequency noises in the photocurrent signal before sending the signal to a preamplifier circuit 40 for amplification.
The amplitude of the reflected light signal received at the photo detector is largely affected by the light-receiving angle at which the photo detector views the reflected light from the symbol being read. The reflected light at the edge of a barcode is thus received less efficiently by the photo detector than light reflected from the center of a barcode. As a result, the signal amplitude and quality is higher for signals received from the area toward the center of the barcode.
Moreover, the cutoff frequency of the low pass filter 30 is conventionally designed as a fixed value and is determined by the frequency of the signal corresponding to the center point A of the barcode 20. However, during the oscillation of the mirror 11, the moving velocity V of the mirror 11 does not remain the same. More specifically, as shown in FIG. 4, the velocity V is at its highest when the mirror 11 arrives at the center of the oscillation (which corresponds to the center point A of the barcode 20), but gradually decreases toward a minimum when it moves to the sides of the angle of αmax. Consequently, the signal response corresponding to the point A has the highest frequency, while the signals corresponding to the points C and D have the lowest frequency. Therefore, the cutoff frequency of the low pass filter 30 is not optimum as to the signals obtained when the mirror 11 is located closer to the sides (i.e., when light beam 15 projects onto the barcode 20 at locations closer to edges A or D). This also decreases the sensitivity in reading the barcode 20.
Therefore, there is a need for a solution to prevent the sensitivity in reading the barcode from decreasing because of the differences in the signal amplitude and/or frequency.